In order to support the communication requirements of high-speed data transmission applications (for instance at bit rates of 25 Gbps) optical links are used as an alternative to merely electrical wire based interconnection. Currently, manufacturers produce products such as modules for optical interconnects and so-called optical cables. In this context, an optical module may be a transmitter (i.e. comprising a light source for transmitting an optical data signal), a receiver (i.e. comprising a photo detector for receiving an optical signal) or a transceiver, which is a combined receiver and transmitter.
Often, modules come with a connector for connecting one or more optical fibers for transporting the optical data signal. In an optical cable the module and fiber is typically pre-connected. It is also possible to have the module or the components of the module mounted directly on a circuit board, such as a motherboard for a computer, for example for interconnects in a supercomputer or as a connection to peripheral equipment.
In the context of the invention an optoelectronic module refers in general to a system comprising optoelectronic components for transmitting or receiving an optical signal connected to driver and/or receiver electronics. Optoelectronic components are in the present context devices arranged to convert electrical energy into optical energy or optical energy into electrical energy, i.e. light sources and photo detectors, such as laser diodes and photo diodes. Often the laser diodes are so-called vertical cavity surface emitting lasers (VCSEL) and as photo diodes p-intrinsic-n photo diodes may be used.
Typically, such a known module will also include an interface allowing the module to be connected to one or more optical fibers as well as control electronics to adjust the operating parameters of optoelectronic components. For example, the operation of a laser diode typically requires an adjustable bias current, modulation current and optionally pre-emphasis. Often, such modules will support more than one channel, such as 2, 4, 8, 12 or 16 channels, but any number of channels is conceivable depending on the application. For such a use the light sources and photo detectors are often available in arrays, such as 1×N arrays or 2×N arrays, wherein N is a positive integer. Strictly, a 2×N array is referred to as a matrix, but in order to simplify notation only the term “array” is used in the following.
In order to convert an electrical data signal into a signal suitable for driving a light source to emit an optical signal comprising this data signal, a driver circuit is required. Similarly, a receiver circuit is required to convert received optical signals into an electrical signal suitable for further transmission in the system. Such driver and receiver circuits are well known in the art and they are typically provided as integrated circuits either as driver chips (comprising driver circuits), transmitter chips (comprising driver circuits), or transceiver chips (comprising a driver and receiver circuit).
A receiver chip may often also be referred as a TIA chip (transimpedance amplifier chip) or a LIA chip (limiting impedance amplifier chip). The chips comprise data pins/pads for receiving/transmitting the electrical data signal to/from a host system and connecting pads for connecting to the optical devices, i.e. connecting pins/pads for connecting to the optical side of the chip (i.e. light sources or photo detectors).
In all data transmission systems, signal integrity is a key issue. Due to the high data rates in recently developed communication systems having data transmission rates of e. g. 25 Gbps, signal integrity, such as for instance the reduction of cross-talk between signal lines, has become a major concern.
Ideally, an interconnection system will carry signals without distortion. One type of distortion is called cross-talk. Cross-talk occurs when one signal creates an unwanted signal on another signal line. Generally, cross-talk is caused by electromagnetic coupling between signal lines. Therefore, cross-talk is a particular problem for high-speed, high-density interconnection systems. Electromagnetic coupling increases when signal lines are closer together or when the signals they carry are of a higher frequency. Both of these conditions are present in a high-speed, high-density interconnection system.
In particular for high-frequency applications, the interconnect structure that is provided to connecting the optical components and the electronic components to each other will have significant impact on the signal integrity. FIG. 1 shows an example of known optoelectronic module with an interconnection between a driver circuitry and a VCSEL array 100 as it might be used for a so-called E/O engine (electro-optic engine) which is suitable for converting electric signals into optical ones. The example of FIG. 1 shows a 12-channel topology with a 12 channel VCSEL array 100 being mounted in an electrically conductive substrate 102. Each of the array's diodes has a cathode and an anode (marked as c and a terminals in FIG. 1) which are connected by means of edge coupled transmission lines or bond wires 104 to a driver circuitry.
In particular, these interconnecting lines are formed in pairs and each pair comprises a signal line 104 (denoted with s) and a ground line 106. As shown in FIG. 1, each of the ground lines 106 are connected to a ground plane layer 108. An example for a technical implementation of this schematic arrangement is given in the Reference Design IPVD12G011-ULM850-10-TT-N0104U rev.04 released Jun. 17, 2009, by Philips Technologie GmbH U_L_M Photonics and IPtronics A/S (to be downloaded from the internet URL http://iptronics.com/files/14774/IPBVD12G011_ULM_RefDesign.04.pdf).
The known module shown in FIG. 1 moreover has an array of further interconnects 110 for connecting a PIN diode array to a belonging amplifier (TIA) also located on the circuit carrier of FIG. 1.
When considering the channel number 7 (marked with reference numeral 112) as a target, and the remaining channels 114, 116 as the transmitter signal aggressors, it can be found that the cross talk, which is generated by this configuration, amounts to 4 ps or 0.1 UI jitter in an EYE diagram at a 25 Gbps bit rate and a bit error rate (BER) of 10−12. Since the VCSEL are placed on a highly conductive substrate, the high coupling and cross talk between neighboring driver VCSEL channels can be attributed to the fact that the impedance of the return current trace is similar to the impedance of a signal trace for the neighboring channel. Thus, significant amount of the aggressor return current is forced to pass through the VCSEL of the target channel, thereby generating a high level of cross talk. This effect is shown schematically in FIG. 2.
Consequently, a problem exists that is to provide an interconnect structure and a belonging optoelectronic module, whereby the signal integrity can be improved in a particularly simple and cost-efficient way.